Mammalian fetal ovary development

Ovarian follicular granulosa cells surround and nurture oocytes, and produce sex steroid hormones. It is believed that during development the ovarian surface epithelial cells invaginate into the ovary and develop into granulosa cells when associating with oogonia to form follicles. Using bovine fetal ovaries (n = 53) we identified a novel cell type, termed GREL for Gonadal Ridge Epithelial-Like. Using 25 markers for GREL and other cells we conducted immunohistochemistry and electron microscopy and chronologically tracked all somatic cell types during development. Before 70 days of gestation the gonadal ridge/ovarian primordium is formed by proliferation of GREL cells at the surface epithelium of the mesonephros. Primordial germ cells (PGCs) migrate into the ovarian primordium. After 70 days, stroma from the underlying mesonephros begins to penetrate the primordium, partitioning the developing ovary into irregularly-shaped ovigerous cords composed of GREL cells and PGCs/oogonia. Importantly we identified that the cords are separated from the stroma by a basal lamina. Around 130 days of gestation as the stroma expands laterally below the GREL cells on the surface thus establishing a sub-epithelial basal lamina and an epithelial-stromal interface, and it is at this stage that a mature surface epithelium develops from the GREL cells. The stroma continues to partition the ovigerous cords into smaller groups of cells eventually forming follicles containing an oogonium/oocyte surrounded by GREL cells, which become granulosa cells. Thus in contrast to the prevailing theory, the ovarian surface epithelial cells do not invaginate into the ovary to form the granulosa cells of follicles.

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Hummitzsch K, Irving-Rodgers HF, Hatzirodos N, Bonner W, Sabatier L, Reinhardt DP, Sado Y, Ninomiya Y, Wilhelm D, Rodgers RJ